1. Field of the Invention
This invention relates to electrodes used in the production of metal in electrolytic reduction cells. More particularly, this invention relates to improvements in the current distribution through the electrode to reduce the electrode temperature and the voltage drop therein.
2. Description of the Prior Art
In the production of metal, such as aluminum, in an electrolytic reduction cell, electrodes are used which are constructed, principally, of conductive material, such as carbon, which will conduct the high currents used for the electrolytic reduction to the molten salt bath in the cells. Carbon electrodes are normally used to avoid contamination of the bath with foreign metals and lower reduction voltage.
The current is normally carried to the electrode by large conductor busses which, in turn, are directly connected to the electrode via a metal rod which, in the case of an anode, also functions as a mechanical support for the anrode as it is lowered or raised in the cell and as a cooling heat sink.
Conventionally, the electrode is attached to the metallic rod by inserting the rod into a central bore formed in the top of the electrode. An electrically conducting ram mix may then be placed into the space between the rod and the bore in the electrode. This connection, however, can be less than satisfactory both from a mechanical standpoint and electrically as well by providing a higher resistance at the interface. This problem has been partially addressed in the prior art. For example, German Pat. No. 1,187,807 discloses a carbon anode having one or more cavities to receive a metal stub or rod. The surfaces of the cavities have grooves or teeth to increase the surface area which is said to provide better conductivity of the current from the rod into the anode.
German Pat. No. 1,937,411 provides for a cast iron structure to be poured around a steel stub placed in the end of a carbon anode. The purpose of the cast iron structure apparently, is to spread the current distribution across the top surface of the anode, as well as to lock the metal rod or stub to the anode by providing an undercutting in the sidewall of the recess cut into the top surface of the anode to receive the molten cast iron. The cast iron, as it solidifies, then provides a dovetail-like fit in the anode to prevent or inhibit the stub from separating from the anode.
Such arrangements do provide better mechanical bonding between the steel support rod and the anode, as well as improving the current distribution in the area immediately surrounding the metal rod or across the upper surface of the anode.
Russian Pat. No. 378,524 illustrates a carbon electrode structure having the usual central bore to receive a metal stub and also having a series of holes drilled into the carbon block parallel to the central bore to receive cast iron rods. Openings are then cut into the carbon between the central bore and the cast iron rods to permit cast iron bridge pieces to be poured to connect the cast iron rods to the metal stub. The purpose of the rods is to reduce power losses.
Despite these attempts to distribute the current more evenly in the anode, there remains a problem, particularly when consumable electrode materials, such as carbon, are used because of the large resistance paths which must be traversed by the current from the metallic current distributing means adjacent the top of the electrode to the bottom of the electrode when the electrode is new. The mere extension of cast iron rods down in the electrode to attempt to bridge some of this distance may be self defeating in that the enhancement of the current carrying ability of the electrode by insertion of such rods, for example, down one third of the length of the electrode, can result in more frequent replacement of the anodes. This is because the electrode, once it is burned off to the point of reaching the metal rod, may need to be replaced if the cast iron, coming into contact with the bath, will result in impurities introduced into the bath. Thus, while such an extension of the current carrying ability would lower the resistance to the bottom of a new electrode, the same result could have been attained by simply shortening the electrode without any change in the useful life since, in both cases, the distance from the bottom of the electrode to the first occurrence of bare iron would have the same path length.
There, therefore, still remains a need for improvement in the current carrying capability of a nonmetallic electrode by better current distribution through the electrode without interfering with the useful life of the electrode by making less of the electrode usable.